1. Field of the Invention
The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) communication system and receiver-transmitters for use in the system as a base terminal and a mobile terminal.
2. Description of Related Art
It is generally known to use the OFDM communication system in MMAC (Multimedia Mobile Access Communication) as an effective transmission system under a multi-pass environment. To improve error rate performance in the OFDM communication system, transmission diversity techniques are proposed in an article entitled “A Study on Transmission Diversity for MMAC System” (SST99-87, pages 87–92), which is published in an technical report of The Institute of Electronics, Information and Communication Engineers.
The essence of the proposed transmission diversity system will be briefly explained with reference to FIG. 10. The system includes a base terminal A having a transmitter-receiver and a mobile terminal B having a transmitter-receiver. Two antennas 1, 2 are provided in the base terminal A. The receiver in the base terminal A receives OFDM signals through both antennas 1, 2. The received OFDM signals are respectively processed by FFT (Fast Fourier Transform) processors 16, 17. This process is a process to transform signals in time-region into signals in frequency-region (referred to as FFT-process). The FFT-processors 16, 17 output a pair of branch signals, respectively. A signal level detector 18 detects the levels of the signals in both branches and determines one branch which has a higher level for each sub-carrier. A selector 21 selects a signal having a higher level for each sub-carrier. A demodulator 22 demodulates selected signals and outputs a series of digital data.
A modulator 23 in the transmitter of the base terminal A modulates signals to be transmitted and feeds the modulated signals to a gain controller 24. The gain controller 24 controls the gain of each sub-carrier signal based on the signal levels detected by the level detector 18, so that the signal levels of all the sub-carriers at the receiving end, i.e., at the mobile terminal B, become equal. The gain-controlled signals for respective sub-carriers are fed to a selector 25. The selector 25 selects a branch (an IFFT processor 29 or 30) having a higher signal level for each sub-carrier based on the information fed from the level detector 18. Each sub-carrier signal is processed in a selected IFFT-processor (Inverse Fast Fourier Transform Processor), 29 or 30, to form OFDM signals. The IFFT-processor 29, 30 converts signals in frequency-region into signals in time-region, and this process is referred to as an IFFT-process. The OFDM signals are transmitted from both antennas 1, 2, respectively, through R-F processors (not shown). In the example shown in FIG. 10, sub-carriers fl and f2 are sent out form the first branch including the IFFT-processor 29 and the antenna 1, while sub-carriers f3 and f4 are sent out from a second branch including the IFFT-processor 30 and the antenna 2.
The receiver in the mobile terminal B receives the OFDM signals sent out from the antennas 1, 2 through a single antenna 101. The OFDM signals received are FFT-processed by an FFT-processor 102 and demodulated by a demodulator 121. On the other hand, signals to be transmitted from the mobile terminal B are modulated by a modulator 122 and IFFT-processed by an IFFT-processor 125, and then transmitted from the antenna 101.
In the proposed diversity system, the error rate performance in the OFDM communication can be improved, since either one of the branches showing a higher transmission performance is selectively used for each sub-carrier. In addition, since the gain of each sub-carrier is controlled at the transmitting end so that signal levels of all the sub-carriers become equal at the receiving end, the error rate performance can be further improved.
However, there is a following problem in the proposed diversity system. The OFDM signals include, in addition to data signals, pilot signals for demodulating the data signals at the receiving end. The phase of the data signals has to be adjusted by the pilot signals. For this purpose, a phase adjuster is provided between the FFT-processor 102 and the demodulator 121, though it is not shown in FIG. 10. The phase adjuster is shown in FIG. 11. The data signals are extracted from the FFT-processed OFDM signals by a data signal extractor 41, while the pilot signals are extracted by a pilot signal extractor 42. A pilot signal generator 43 generates reference pilot signals that have the same amplitude and phase as those of the pilot signals of the transmitting end. A phase-rotation calculator 44 calculates an amount of phase-rotation in the pilot signals received based on the reference pilot signals fed from the pilot signal generator 43. A phase adjuster 45 adjusts the phase of the data signals received using the calculated amount of phase-rotation.
In the proposed diversity system, the data signals and the pilot signals are sent out from either one of the branches selected for each sub-carrier. More particularly, as shown in FIG. 12, each data signal (shown with a thin line) of a given sub-carrier is sent out from either one of the branches (an atenna 1 or 2) selected for that sub-carrier, and each pilot signal (shown with a thick line) of a given sub-carrier is sent out from one of the branches (an antenna 1 or 2) selected for that sub-carrier. If the signals under the same sub-carrier are sent from both antennas, the signals interferes with one another. To avoid the interference, a signal of each sub-carrier is transmitted from only one antenna.
As shown in FIG. 12, the OFDM signals sent out from the two antennas 1, 2 are received by a single antenna 101, FFT-processed by a FFT processor 102. Then, the data signal phase is adjusted by a phase adjuster 103, and the data signals are demodulated by a demodulator 121. That is, the signals sent out from two antennas through different routes are received by one antenna and handled as if the signals were a single signal. Since the signals transmitted through different routes have respectively different phase-rotations, the phase adjustment cannot be done accurately if the signals are handled as a single signal.